This invention relates to radio communication systems. More particularly, and not by way of limitation, the invention is directed to an arrangement and method for reducing the impact of an interfering signal in a communication system. The preferred embodiment is described in terms of a radio system utilizing Orthogonal Frequency Division Multiplexing (OFDM).
The number of transceivers in devices such as mobile phones, personal digital assistants (PDAs), laptops, and the like is increasing at the same time as many of these devices are becoming smaller. This means that it is becoming more important that these different radio frequency (RF) systems co-exist without seriously degrading the performance of other systems. Several of the emerging technologies that are expected to be found in mobile phones and similar devices in the near future are using OFDM to increase the number of users within a given frequency band. For example, OFDM is used for Wireless Local Area Networks (WLAN), Broadband Access (Wi-Max), and Digital Broadcasting (DVB-T, DVB-H, DAB). OFDM has also been proposed for the next generation (4G) of cellular networks.
OFDM is especially suitable for situations where the channel is highly time-dispersive. Time-dispersion causes part of the OFDM symbol to be corrupted due to inter-symbol interference (ISI). OFDM systems, therefore, are typically designed with some amount of redundancy in the received signal, with part of the received signal being removed by the receiver prior to further processing. If the received signal is processed properly, the part of the signal corrupted by ISI does not have any impact on the overall performance.
FIG. 1 is an illustrative drawing of three symbols (Symbol k−1, Symbol k, and Symbol k+1) in a conventional OFDM signal. Each symbol includes a first portion referred to as a guard interval (GI) 11. The GI is transmitted over a time interval Tg. The GI is followed by other information 12 transmitted over a time interval Tu. At the end of each symbol is a portion 13 of the other information in which the information in the GI is repeated. The purpose of the GI is to ensure that the there is no ISI between the “actual” symbols. The GI is sometimes referred to as a Cyclic Prefix (CP).
FIG. 2 is a simplified flow chart of a conventional OFDM transmission process. At step 15, an inverse fast Fourier transform (IFFT) is used to modulate the information signals to form the symbols. At step 16, the last portion of each symbol is copied and added to the beginning of the symbol to form the GI. Further processing not relevant to the present invention is then performed at step 17 to form the transmitted signal 18.
FIG. 3 is a simplified flow chart of a conventional OFDM reception process. The transmitted signal 18 is received, and at step 21 front-end processing not relevant to the present invention is performed. At step 22, the receiver removes the GI 11. At step 23, an FFT is used to demodulate and recover the information signals. As shown in FIGS. 2 and 3, one can view OFDM as if the signal is generated in the frequency domain, transformed to the time domain by the IFFT, transmitted in the time domain, transformed back to the frequency domain by the FFT, and then further processed.
FIG. 4 is an illustrative drawing of a conventional windowing process for removing the GIs from a series of received symbols prior to the FFT. By setting a window value to one (1) during an FFT window 25, and setting the window value to zero (0) during the GI 11, the information in the GI is discarded.
FIG. 5 is an illustrative drawing of a series of conventional received symbols and a strong interfering signal 28, which is present for only a small fraction of each FFT window 25. The interfering signal is intermittent in time, and is not synchronized to the OFDM signal. For example, the interfering signal may be a Bluetooth signal and the duration of the bursts may then be approximately 300 μs. The OFDM signal, on the other hand, may be a DVB-H signal, in which case the total duration of each symbol, including the GI, is about 1.1 ms. As illustrated in FIG. 5, the interfering signal 28 interferes with the GI of Symbol k−1, the middle portion of Symbol k, and the last portion of Symbol k+1. Since the GI is discarded by the receiver, the interfering signal will have no effect on Symbol k−1, whereas the quality of the two other OFDM symbols may be degraded.
Since OFDM symbols have relatively long duration, the probability that some portion of the symbol will be interfered with becomes relatively large even if the interfering signal is only present for a small fraction of the symbol duration. Also, if the interfering signal is very strong, then only a small part of the OFDM symbol needs to be disturbed in order to significantly degrade the performance of the OFDM system. Additionally, OFDM systems are typically designed to handle certain channel conditions such as maximum delay spread and maximum Doppler, but are not designed to handle strong interference caused by the receiver being co-located with a transmitter of another system. Therefore, even an interfering signal of short duration and low power may seriously degrade the OFDM performance.
Problems may also be encountered if link adaptation is employed. With link adaptation, coding and modulation are adapted based on the estimated channel conditions. If a co-located system interferes with the OFDM system, the system may erroneously conclude that the communication between the transmitter and the receiver is poor. As a result, the coding and modulation may be upgraded accordingly. The algorithms for link adaptation are typically developed based on the assumption that interference is due to other users of the same type of RF system (e.g., other OFDM users). Therefore, the algorithms may malfunction when the interference is caused by different, co-located RF systems.
What is needed in the art is an arrangement and method for providing robustness in communication systems that overcomes the shortcomings of the prior art. Such a system and method should reduce the impact of interference from interfering systems external to the device in which the communication system is operating and from interfering systems co-existing within the same device. The present invention provides such an arrangement and method.